Solid propellants of enhanced burning rates using bimetallic fibers



United States Patent O U.S. CL 102-102 6 Claims ABSTRACT F THE DISCLOSURE A method for increasing the effective burning rates of a solid propellant by dispersing in the solid propellant a plurality of bi-metallic fibers, so that the longitudinal axis of each fiber is perpendicular to the plane of the burning surface of the solid propellant.

This invention relates to improvements in methods for increasing the effective burning rates of solid propellants, and more particularly it relates to the use of bi-metallic fibers that are dispersed in a solid ypropellant so that the longitudinal axis thereof is perpendicular to the plane of the burning surface of the solid propellant.

It is a well-known fact that the burning rate of solid propellants, in which metallic fibers are deposited are dependent as to their burning characteristics on the geometry of the metal fibers as well as the concentration and composition thereof. Mono-metallic fibers made from silver or copper produce higher burning rates than monofibers made from aluminum or zirconium, but the monometallic fibers made from silver or copper have a tendency to decrease the specific impulse of the solid propellant. To overcome this disadvantage the possibility of using bi-metallic fibers to increase the burning rate of the solid propellant without any significant decrease in the specific impulse of the solid propellant, was determined from thermochemical calculations. The bi-metallic fibers considered were of uniform cross-section and comprised two different metals that were in physical contact with each other. Copper, zirconium, titanium and beryllium were each combined with aluminum to provide the bimetallic fibers.

It was determined that the bi-metallic fibers produce higher propellant burning rates: that the largest increases in burning rates of the solid propellant were achieved with zirconium-aluminum fibers; that beryllium-aluminum fibers increase both the specific and density impulse of the solid propellant and further, that titanium-aluminum and zirconium-aluminum bi-metallic fibers resulted in only slight reductions in the specific impulse of the solid propellant.

Another step that was taken in producing high burning rate solid propellants, was the addition of chemical burning rate catalysts, as well as, the addition vof mono-metallic fibers. The combination of chemical burining rate catalysts and mono-metallic fibers achieved high burning rates without sacrificing either specific impulse or density. It was also found that the high burning rate and high specific impulse was achieved by using mono-metallic fibers that were composed of either aluminum or zirconium.

As previously stated, mono-metallic fibers had been tried, but copper and silver mono-metallic fibers each have a serious drawback. The copper mono-metallic fibers degrade the specific impulse of the solid propellant more than the mono-metallic fibers composed of aluminum and the cost of silver mono-metallic fibers is prohibitively expensive. For these reasons neither mono-metallic fibers composed of copper or silver have been extensively used ICC in solid propellants because of the penalties resulting in the use of mono-metallic fibers composed of copper in specific impulse and because of the prohibitive cost of using mono-metallic fibers composed of silver.

It has also been found that the intrinsic ability of a mono-metallic fiber to enhance the burning rate of the solid propellant depends primarily upon the thermal diffusivity and melting temperature of the individual monometallic fiber. Therefore, to achieve maximum enhancement of the burning rate of the solid propellant, both the melting temperature and the thermal diffusivity of the individual mono-metallic fibers should be as high as found to be feasible. However, it was founded that monometallic fibers that possessed the required high thermal difusivity had only moderate melting temperatures, and that mono-metallic fibers having high melting temperatures possessed only moderate values of thermal diffusivity.

It was determined, therefore, that to achieve maximum results in enhancing the burning rate of a solid propellant, the use of bi-metallic fibers should be used, one of the metallic fibers to possess a high melting temperature and the other metallic fiber to possess high thermal diffusivity, thus higher burning rates could be achieved than were obtainable by using mono-metallic fibers in the solid propellant.

To be effective in enhancing the burning rates of the solid propellant, the bi-metallic fibers must have a uniform cross-section and be composed of two different mono-metallic fibers which have substantially the same rectangular cross-sections and are in intimate physical contact along one longitudinal side thereof. Accordingly, the bi-metallic fibers were composed of copper-aluminum, titanium-aluminum, zirconium-aluminum and berylliumaluminum.

It had been found that the burning rate of the solid propellant having mono-metallic fibers dispersed therein, depended on the concentration of the mono-metallic fibers in the solid propellant. It was also determined that the average burning rate along a mono-metallic fiber embedded in a solid propellant represents the upper limit of the burning rate that can be achieved by dispersing the mono-metallic fibers throughout the solid propellant, since the upper limit of the burning rate represents the ultimate rate enhancing capability of a particular mono-metallic fiber.

It is also known that the burning along the length of a mono-metallic fiber occurs in three phases:

(l) An initial transient phase, in which the propellant burning rate along the mono-metallic fiber increases from the burning rate of the basic solid propellant to a higher rate.

(2) A steady-state phase, in which the burning rate along the mono-metallic fiber remains essentially constant.

(3) A final transient phase, in which the burning rate along the mono-metallic fiber increases rapidly to large values as the burning surface approaches the bottom or lower end of the mono-metallic fiber.

The preceding action, as carried out in the three phases, indicates that the mono-metallic fibers functions as a lowresistance path for the transfer of heat from the hot products of combustion to the solid propellant beneath the burning surface thereof. This would tend to indicate that the transfer of heat from the mono-metallic fiber to the solid propellant increases the temperature of the solid propellant adjacent to the mono-metallic fiber, thereby increasing the burning rate of the solid propellant adjacent to the mono-metallic fiber. Thus the increase in burning rate in the solid propellant immediately surrounding the mono-metallic fiber results in an overall increase in the burning rate of the solid propellant.

The foregoing analysis, therefore, indicates that the acceleration of the burning rate achieved by the use of mono-metallic fibers is correlated with the thermophysical properties of the mono-metallic fiber.

If, therefore, the burning rate of a mono-metallic fiber is thermally controlled, the use of a bi-metallic liber, wherein one of the metallic fibers possesses a high melting temperature and the other metallic fiber possesses a high thermal diffusivity, would produce a higher burning rate than the use of a mono-metallic fiber. This result would be obtained because the metallic fiber having the high melting temperature would project relatively far into the combustion gases, thereby providing a large area for heat transfer to the metallic fiber while the metallic fiber composed of a high diffusivity material would provide for rapid conduction of the heat provided thereby to the subsurface regions of the solid propellant.

The bi-metallic fiber embodying the instant invention will be composed of metallic fibers having different melting temperatures. Therefore, the three general phases for a mono-metallic fiber will not prevail where a bi-metallic fiber is concerned, but rather would change to a four phase procedure as follows:

(1) An initial step in which the solid propellant burns away from the exposed end of the bi-metallic fiber and the combustion gases created by the burning of the solid propellant heats the upper end of the bi-metallic fiber.

(2) A second step in which the metallic fiber possessing the lowest melting point reaches its melting temperature and separates from the metallic fiber having the higher melting point.

(3) A third step in which the temperature of the bimetallic fiber has increased to the point Where the metallic fiber having the higher melting point begins to melt.

(4) The steady-state phase in which the melting rates of both of the metallic fibers and the burning rate along the entire bi-metallic fibers are equal. x

It is an object of this invention, therefore, to provide a bi-metallic fiber wherein the melting point of one of the metallic fibers is lower than the melting point of the other metallic fiber.

It is another object of this invention to provide a bimetallic fiber wherein one of the metallic fibers possesses a high degree of thermal diffusivity.

It is a further object of this invention to provide a bimetallic fiber wherein one of the metallic fibers may be composed of zirconium, titanium and beryllium and the remaining metallic fiber composed of aluminum.

It is a still further object of this invention to utilize bimetallic fibers that are substantially uniform in size in cross-section and are in intimate contact with each other.

Other objects and advantages will become obvious from the following detailed description.

In the drawing:

FIG. 1 is a fragmentary sectional view showing in elevation a bi-metallic fiber embodying the present invention dispersed in a solid propellant;

lFIG. 2 is a fragmentary sectional view similar to FIG. 1 wherein both of the metallic fibers of the bi-metallic fibers embodying the invention have reached their melting f points forming a substantially conical burning surface adjacent to the bi-metallic fibers;

FIG. 3 is a fragmentary sectional view similar to FIG. 2 wherein the melting rates of both of the metallic fibers of the bi-metallic fibers are equal and the solid propellant is reaching its burn out point;

FIG. 4 is a fragmentary View looking down on the burning surface of the solid propellant on the line 4--4 of FIG. 2.

In FIG. 1 the bi-metallic fiber 10 embodying the invention is shown to be dispersed in a fragment of solid propellant 11. The bi-metallic fiber is formed from a metallic fiber 12 that is composed of aluminum and a metallic fiber 13 that may be composed of zirconium, titanium or beryllium.

An analytical solution of the performance of the burning of the solid propellant along the bi-metallic liber was obtained by careful calculations of the solid propellant being consumed and Iby moving pictures taken during the burning of the solid propellant.

In order, therefore, to simplify the mathematical analysis of the instant invention the following deductions were arrived at:

The bi-metallie fiber 1l)` must be thermally thin, the solid propellant `11 must be homogeneous and isotropic and all the thermophysical properties of the bi-metallic fiber 10 and the solid propellant 11 must be constant. The gas fiow in the cone 14 surrounding the bi-metallic fiber 10 must be quasi-steady, non-viscous and incompressible and the temperature distribution in the solid propellant 11 and the ow patterns in the cone 14 around the bi-metallic fiber 10 must be two-dimensional.

The effect of a bi-metallic fiber having metallic fibers of i different compositions, was to produce higher burning Metallic fiber materials: Average burning rate in secs.

Aluminum 4.7 Zirconium 4.1 Beryllium 3.0 Titanium 2.2

Burning rates for bi-metallic fibers: Metallic fiber materials: Average burning rate in secs.

Aluminum and zirconium 7.8 Aluminum and beryllium 5.6 Aluminum and titanium 7.7

It is evident, therefore, that the use of a bi-metallic fiber greatly increases the average burning rate of the solid propellant.

Calibrated tests, therefore, proved that the burning rates achieved by the bi-metallic fibers far exceeds the burning rates achieved by a mono-metallic fiber.

The thermochemical results obtained by the use of a bi-metallic fiber indicate that higher burning rates are achieved without significant degradation in specific impulse that are the result of the use of monometallc fibers.

It has also been determined that the enhancement of the burning rates of a solid propellant depend on the length of the bi-metallic fibers, for the longer the bi-metallic fibers are, the greater is the enhancement of the burning rates ot' the solid propellant. i

The metallic fibers of the bi-metallic fiber embodying the invention are retained in intimate contact with each other by several methods, namely, flame spraying using the Plasmatron, electro plating and rolling the metallic fibers in an inert atmosphere in order to obtain an intermolecular bond. The metallic fibers must be so bonded that the molten `metal or oxide of one will not interfere with the burning of the other. Coating one metallic ber with the other to completely surround one of the metallic fibers is not a successful procedure, for the melting of the coated metallic fiber degrades the burning rate enhancement of the metallic fibers because the molten metallic lfiber remains in contact with the other metallic liber before it reaches its molten state. Thus, the separate metallic fibers embodying the invention do not detract from each other during the period they are consumed by the burning solid propellant. The bimetallic fibers are dispersed in the solid propellant so that the longitudinal axis of the bimetallic fibers are substantially perpendicular to the burning surface of the solid propellant and since aluminum, one of the metallic fibers used to produce the bi-metallic fiber, is an excellent heat conductor, it causes the solid propellant below the burning surface to be preheated and since the other metals aforementioned have relatively high melting temperatures, such metals function as heat fins to conduct heat created by the combustion gases directly to the aluminum metallic fiber, thus, greater heat is conducted by the aluminum metallic `fiber to the solid propellant.

It has been definitely established, therefore, that bimetallic fibers enhance the burning rates of the solid propellant more than do most monofilament metallic fibers and it is believed that from the foregoing description, the manner in which the bi-metallic fibers are utilized for increasing the burning rates of solid propellant will be clear to those skilled in the art and any variations in the invention is believed to be fully covered, providing such variations fall within the spirit of the invention and the scope of the appended claims.

Having thus described the invention what is claimed as new and desired to be secured by Letters Patent is:

1. A method for enhacing the burning rates of solid propellant which includes the step of dispersing in a solid propellant, bi-metallic fibers and wherein each of the metallic fibers possess dierent melting temperatures.

2. A method for enhancing the burning rates of solid propellants which includes the step of dispersing in a solid propellant bi-metallic fibers composed of aluminum and zirconium.

3. A method for enhancing the burning rates of solid propellant which includes the step of dispersing in a solid propellant bi-metallic fibers composed of aluminum and titanium.

4. A method for enhancing the burning rates of solid propellant which includes the step of dispersing in a solid propellant bi-metallic fibers composed of aluminum and beryllium.

5. A method for enhancing the burning rates of solid propellant which includes the step of dispersing in a solid propellant bi-metallic fibers so that the longitudinal axis of the bi-metallic fibers are perpendicular to the burning surface of the solid propellant.

`6. In a solid propellant grain having a burning surface, that extends in a plane transversely of the longitudinal axis thereof, the improvement comprising the combination with said solid propellant, of a plurality of bi-metallic fibers embedded in said solid propellant to enhance the burning rates of the burning surface of said solid propellant, each of said bi-metallic fibers including first and second metallic fibers, with each of said first and second metallic fibers being of rectangular shape, of substantially equal size in cross section and bonded in intimate contact with each other along their adjacent longitudinal surfaces, the first of said metallic fibers having a higher degree of thermal diffusitivity than the second of said metallic fibers, and the second of said metallic fibers having a higher melting temperature than the first of said metallic fibers and the bi-metallic fibers are embedded in said solid propellant, so that the longitudinal axis of each of said Ibimetallic fibers is perpendicular to the plane of the burning surface of said solid propellant.

References Cited UNITED STATES PATENTS 3,107,620 10/1963 ODonnell 102-98 3,109,374 11/1963 |Rumbel et al. 102-98 3,109,375 1l/l963 Rumbel et al 102-98 3,128,706 4/ 1964 Rumbel 102-98 3,163,113 12/1964 )Davis et al 102-98 3,140,663 7/1964 Rumbel et al. 102-102 r BENJAMIN R. PADGETT, Primary Examiner 

